The invention relates to an apparatus for and a method of providing controlled modulation of the high frequency AC current of a high-intensity discharge (HID) lamp. Specifically, the low frequency ripple voltage remaining on the DC bus voltage after its conversion from an AC line is utilized as a modulation input signal to a controller, which adjusts the switching frequency of the inverter.
The electronic ballast is one of the most cost sensitive products of power electronics. Its cost is a major factor constraining market penetration of electronic ballasts for HID lamps.
One possible solution to this problem is to utilize a high frequency HID electronic ballast such as that described in U.S. Pat. No. 6,181,076 issued to Trestman et al. High frequency electronic ballasts cost less than corresponding low frequency electronic ballasts because the high frequency ballasts includes fewer parts. This is due to the fact that at higher operating frequencies it is possible to use resonant ignition, thereby eliminating a separate igniter circuit. Additionally, the increase in operating frequency is accompanied by a corresponding decrease in the physical size and cost of the ballast, because the reactive components such as capacitors and inductors are both smaller and less expensive.
This general relationship is true only up to a point, however, because above a certain frequency the cost of the ballast begins to increase with increased frequency. There are several reasons for this:
One of the most significant factors determining ballast cost is the cost of the controller and the driver for the switch or switches of the inverter. These switches are most often implemented as field effect transistors (FETs). At present, there are several inexpensive electronic ballast control ICs. Such devices integrate a half bridge inverter controller with low side and high side FET drivers, a startup circuit, and a fault protection logic circuit. One example of such a device is the ST Microelectronics L6574.
Other electronic ballast control ICs also include a power factor correction (PFC) controller with a PFC FET driver. Examples of such a device are the ST Microelectronics L6570 and the International Rectifier IR2167. These control ICs can operate at frequencies up to 350 kHz. In order to achieve higher operating frequencies, it is necessary to use special much more expensive high frequency controllers and separate high frequency FET drivers, both of which contribute to increased overall cost.
The common inexpensive FETs have a comparatively slow body diode. Above an operating frequency of approximately 400 kHz, losses in such diode became unacceptably high. As a result, in order to operate the ballast at these higher frequencies, either two high frequency diodes should be added for each transistor, or specially designed, and hence more expensive, FETs with fast body diode should be used.
As an additional factor, when operating frequencies are above 350 kHz it is necessary to use more expensive ferrite core materials and more expensive Litz wire for the magnetic components.
FIG. 1 is a graph illustrating ballast cost as a function of operating frequency reflecting the points considered above. As illustrated therein, minimum cost is achieved at a frequency of approximately 300 kHz.
A further consideration which must be borne in mind in lamp and ballast design is the wide spectrum of standing acoustic waves.
The acoustic spectrum for a Sylvania MPD39PAR30LN/U/830/FL 39W HID metal halide lamp is illustrated in FIG. 2. Each peak corresponds to a certain standing acoustic wave in the discharge vesselxe2x80x94a resonant component of the acoustic spectrum. The magnitude of the resonance component represents it strength. The stronger a component, the less damped it is and therefore the more easily it could be established and sustained. It is very difficult to stabilize an arc in an area with the presence of strong resonance.
As the size of a lamp burner decreases, its acoustic spectrum widens. For example, the acoustic spectrum of a 70W HID lamp with a quartz burner does not contain harmonics above 150 kHz, and the arc is stable if the lamp is driven by a 300 kHz ballast. As seen from the acoustic spectrum of a 39W lamp with a ceramic burner illustrated in FIG. 2, there are harmonics with a small magnitude (weak harmonics) in the area of 300 kHz. If one were to use a 300 kHz ballast to drive such a lamp, acoustic waves would arise and as a result the arc would not be stable.
One known method of arc stabilization utilizes frequency modulation of the AC current. It has been determined through experimentation that such modulation reliably stabilizes the arc in the range of weak resonance.
There exist a number of patents related to ballasts operating at high frequency with frequency modulation. Example circuits are described in U.S. Pat. No. 5,680,015 issued to Bernitz at al. and U.S. Pat. No. 5,859,505 issued to Bergman at al. In the circuits described by these patents, the ballast includes such highly complex electronic components such as microprocessors, analog-to-digital (A/D) converters, etc. These circuits are designed to detect arc instabilities, store date in the microprocessor memory, calculate a stability factor and adjust the operating frequency so as to work at a frequency with minimized arc instability. This complex circuitry significantly increases the cost of ballast.
Another prior art reference is U.S. Pat. No. 5,923,128 issued to Canoga. The circuit includes a triangular current wave generator specifically added to the ballast to provide a triangular waveform that modulates the frequency of the switching signal controlling the inverter. The ballast described by this patent is intended to operate at a frequency of approximately 20 kHz.
The present invention provides an electronic ballast for operating a high intensity gas discharge (HID) lamp from an AC line. The electronic ballast includes an EMI filter, a bridge rectifier, a power factor corrector (PFC), an inverter and an inverter controller. The inverter circuit provides the HID lamp with AC current within a frequency range of 250-350 kHz.
There exist a number of inexpensive components including field effect transistors (FETs), controller chips with built-in high side and low side FET drivers, ferrite cores, etc., that are designed to operate in this frequency range. Use of these components minimizes the cost of the ballast.
In the frequency range of 250-350 kHz, acoustic resonance harmonics in the lamp are either absent, as in the case of HID lamps with large burners, or their magnitudes are small, as in the case of HID lamps with small burners. The electronic ballast of the present invention provides frequency modulation of the output AC current such that deleterious effects of the lamp""s acoustic resonance are eliminated.
To simplify the circuitry, voltage ripple on the bulk capacitor of the PFC output (DC bus) is used for frequency modulation. The modulation range of the switching frequency is determined by choosing the value of a small capacitor connected between the positive electrode of the bulk capacitor and a frequency set pin of the inverter controller. With such an arrangement, the only additional circuit component is a small capacitor, which results in virtually no increase in the cost of the ballast.
The ballast of the present invention is designed to operate at a frequency in the area of 300 kHz, where the cost of a ballast is minimal, as illustrated in FIG. 1. In this frequency range, the magnitude of acoustic waves, even in the case of a lamp with a small burner, are small and can be suppressed by frequency modulation of the lamp current.
To achieve frequency modulation of the lamp current, the present invention makes use of the voltage ripple on the DC bus (120 Hz in the case of a 60 Hz line frequency) to supply the modulating signal.
To convert the AC ripple voltage to AC current, the disclosed ballast utilizes the impedance of an additional capacitor connected between DC bus and the Freq. Set pin of the inverter controller. The total current flowing into the Freq. Set pin is the sum of the current flowing through this capacitor and the frequency set current, the combination of which provides low frequency modulation (e.g. 120 Hz) of the high frequency signal (e.g. 300 kHz) of an oscillator internal to the inverter controller. The capacitor connected between DC bus and the Freq. Set pin also serves a DC blocking function.
It is an object of present invention is to provide a much simpler and less expensive circuit for HID lamp current modulation.
Another object of the present invention is to provide an apparatus and method to utilize the existing AC ripple on the DC bus voltage as a modulating signal.
It is a further object of the present invention to provide a method and apparatus to connect DC bus voltage having an AC ripple to a frequency set input of an inverter controller in a ballast.
It is yet another object of the present invention to provide a method and apparatus to connect a DC bus voltage with low frequency ripple voltage to a frequency set input of an inverter controller through a reactive component.
It is yet a further object of the present invention to provide a low-cost ballast circuit operating at a switching frequency outside of regions of strong resonance.